“Diesel fumes trigger heart attacks and strokes”, The Independent reported. It said researchers had “discovered how air pollution triggers heart attacks”. The Metro reported that “car exhausts can cause heart stress levels to rise threefold”. The Independent added that this “may account for the rise in heart deaths on days when pollution from traffic fumes is high”.

This report is based on a trial that looked at a small number of men who had suffered previous heart attacks and monitored their response to exercise when breathing in dilute fumes or filtered air. However, this study provides little evidence to support the theory that traffic fumes increase the risk of heart attacks; much further research would be needed to establish whether there was a link between the two.

Where did the story come from?

This research was conducted by Dr Nicholas Mills and colleagues from the Centre for Cardiovascular Science and the Centre for Inflammation Research at Edinburgh University, and Departments of Respiratory Medicine and Allergy, and Medicine, at Umea University, Sweden. The study was funded by a Michael Davies Research Fellowship from the British Cardiovascular Society, the British Heart Foundation, and various grants from Swedish research programmes. It was published in the peer-reviewed medical journal New England Journal of Medicine.

What kind of scientific study was this?

This was a small randomised controlled crossover study, where the participants were randomly exposed to both diesel fumes and a placebo: filtered air.

The researchers used 20 male volunteers,who had suffered a previous heart attack that had been treated by unblocking the affected heart artery with a balloon and inserting a tube to keep the artery open (angioplasty and stenting).

The tests were carried out in two, 8am sessions, with at least two weeks in between sessions. The participants were placed into a chamber containing either filtered air or dilute diesel exhaust, for one hour. During that time they cycled for two 15 minute periods on a bike, with rest periods in between. Recordings of the participant’s heart rate and rhythm pattern were taken by ECG (electrocardiograph) during the experiment.

As an additional part of the experiment, six hours after exposure the participants were injected with medication to dilate the blood vessels and blood samples were taken to look at the amounts of enzymes in the blood which dissolve blood clots and other markers of inflammation.

What were the results of the study?

The researchers found that during exercise the heart rate of the participants increased both when they were breathing filtered air and when they were breathing exhaust fumes in concentrations that commonly occur in urban road traffic. During exercise there was also evidence of a strain upon the heart from a lack of oxygen during both types of exposure; however, the level of the strain on the heart was significantly greater during the exposure to diesel fumes. In one particular measurement on the ECG, this was three times greater during diesel exposure than during filtered air exposure. There was no difference between the groups at rest.

The results of blood samples found that the levels of enzymes in the blood that dissolve blood clots were significantly less following exposure to fumes.

What interpretations did the researchers draw from these results?

The researchers conclude that brief exposure to dilute diesel exhaust fumes in men with previous heart disease increases exercise-induced strain upon the heart muscles and reduces the ability to dissolve blood clots. They say that their findings point to “mechanisms that may explain in part the observation that exposure to combustion-derived air pollution is associated with adverse cardiovascular events”.

What does the NHS Knowledge Service make of this study?

Although this study has provided positive evidence of extra strain on the heart when heart attack victims exercise while exposed to diesel fumes, it has several limitations and further evidence is needed before a conclusive link could be proved.

These participants were all middle-aged men who had previously suffered from heart attacks. It is not possible to generalise any findings to those without any history of heart disease, or to younger people, as the paticipants are likely to be at much higher risk of a further heart attack during exercise already.

Signs of strain on the heart were present during exercise while breathing both filtered air and exhaust fumes; it cannot be assumed that these men would have gone on to have a heart attack when breathing fumes and that when breathing air they would not.

This was also a very small trial of only 20 men.

The study gave positive results only for a particular effect of diesel fumes upon the heart during exercise. This study could only suggest a link between exercising while exposed to fumes in at-risk men, not just that ‘air pollution triggers heart attacks’ as the newspapers reported.

This study only included brief exposure; no conclusions on the longer-term effects of air pollution can be drawn.

Air and traffic pollution is made up of many different chemical particles; the fumes used in this experiment may not be completely representative of the air outside, and therefore the effects may not always be the same.

Lastly, this study did not look into the risk of stroke at all. There is nothing to suggest, as was reported, that ‘diesel fumes trigger strokes’.

Much further research in many more individuals with past heart attacks would be needed to establish whether there was any added risk from exercising in traffic pollution, in just this high-risk group. The effects of traffic fumes both upon individuals with risk factors for heart disease, or healthy young people at very low risk have yet to be examined.

There are proven large and long-term benefits to maintaining or increasing physical activity (to moderate levels) following a heart attack. These need to be balanced against any temporary risks that future studies like this may show.

Sir Muir Gray adds...

Pollution is bad for the individual and for the planet; individuals whose bodies are already affected by disease are more susceptible to the effects of air pollution from cigarettes or traffic.